1. Field of the Invention
This invention relates to terminal block assemblies of the type including a plurality of electrically-isolated sections, each having a terminal located therein. In particular, the invention relates to terminal block assemblies having dead-front protection. With this type of protection the terminals located in the sections are at least partially covered, so that shock and short circuit hazards are minimized. Such protection is not only desirable but for certain applications is required under UL Standards in the United States and under VDE Standards in Western Europe.
2. Description of the Prior Art
Terminal blocks which provide dead front protection are described in U.S. Pat. No. 4,180,305, having the same applicants as the present application, and in U.S. Pat. No. 3,247,480. Each of these terminal blocks includes a cover which snaps over the top of barriers between adjacent terminals. The cover must be removed or pivoted away from the terminals, however, to gain access to the terminals, and dead-front protection must therefore be temporarily sacrificed. Protection will be lost if the cover is misplaced or intentionally removed.
A portion of a conventional dead-front terminal block assembly which provides constant protection is illustrated in FIG. 1. This terminal block assembly consists of a one-piece insulating molding 10 having a plurality of hollow sections 12, each containing a tubular clamp terminal 14. Each terminal comprises a tubular conductor 16, open at both ends, a pair of clamping screws 18 mounted in respective threaded holes formed in one side of the tubular conductor adjacent the open ends, and a clamping strap 20 passing through the tubular conductor near the side having the threaded holes. Each section 12 of the insulating molding 10 has openings 22 at opposite ends providing access to the opposite ends of the tubular conductor 16 located in the section, for enabling the insertion of wires or other conductive elements which are to be electrically-connected to each other by the tubular conductor. The insulating molding 10 also includes hollow extensions 24 for enclosing the heads of the clamping screws 18. The end of each hollow extension has an access opening 26 for inserting a screwdriver or test prod. The screws 18 in each tubular conductor 16 are turned, after insertion of wires (not shown) into opposite ends of the tubular conductor, forcing the ends of the respective clamping strap 20 against the wires to clamp them in place.
In dead-front terminal blocks such as that shown in FIG. 1, the openings for providing access to the tubular conductors and the openings for providing access to the screw heads are perpendicular to each other. Thus, complex molds having mutually perpendicular projections are needed to form the openings. Typically these molds comprise separable halves which together form a cavity for receiving molding material. The mutually perpendicular projections typically include fingers extending into the cavity from at least one half of the mold, toward the other half, and removable core pins inserted through side walls of the mold and extending into the cavity in a direction perpendicular to the fingers. Because of their complexity, such molds are both costly to produce and to operate.
Even with complex molds, dead-front terminal blocks cannot be produced which accept many of the terminal types commonly used with terminal blocks, such as right angle feed-through terminals. These types of terminals cannot be inserted through the small, limited-access openings provided in dead-front terminal blocks.
It is also difficult to assemble the tubular clamp terminals used with conventional dead-front terminal blocks, because the clamping screws cannot be screwed into the threaded holes of the terminals' tubular conductors until both the clamping screws and the tubular conductors are inserted into the hollow sections of the block through their respective access openings. After insertion, the ends of the screws must be aligned with the threaded holes, while both are hidden from view by the insulating block material. Removal of terminals found to be defective after assembly (such as those having damaged threads) is also difficult, because the hollow extensions enclosing the heads of the screws typically include means such as inwardly-extending projections for holding the screws captive after they are forcefully inserted past the projections.
In terminal blocks utilizing tubular clamp terminals it is desirable to have a wire stop in each section for preventing a wire or other conductor, inserted into one end of the terminal in the section, from passing more than halfway through the terminal's tubular conductor, and thus inhibiting insertion of a conductor into the opposite end. Typically such stops have been formed in either the tubular conductor itself or the clamping strap, thus complicating the structure of the terminal.